If you integrate all the bbPress installs to use the same users table, then you can let your users sign in even if your bbPress installs are on separate domains… if you use ck’s awesome “Cross Cookie” plugin:

Definitely backup all your data before playing around… it should be pretty safe to have multiple installs use the same user table.

If you hit a snag, post and let us know and I’ll see if I can help. I integrated multiple bbpress installs of 0.9 not 1.0, but if it’s not too different maybe I can help. I’ve got almost 10 bbPress installs working together with a single users database, so there’s definitely a path!